A volume holographic recording system is known as a digital information recording system employing the principle of hologram. The feature of the system is that an information signal is recorded in a recording medium as a change of a refractive index. The recording medium is made from a photo-refractive material such as single-crystal lithium niobate.
As one of the conventional holographic recording and reproducing methods, a method is known which performs recording and reproduction by using Fourier transform.
As shown in FIG. 1, in a conventional 4f-system holographic recording and reproducing apparatus, laser light 12 emitted from a laser light source 11 is split into a signal light 12A and a recording reference light 12B by a beam splitter 13. The signal light 12A passes through a beam expander 14 where the beam diameter thereof is expanded, and is then incident as collimated light on a spatial light modulator (SLM) 15 such as a transmission-type TFT liquid crystal device (LCD) panel. The spatial light modulator (SLM) 15 receives recording data that has been converted into an electric signal by an encoder 25, and forms a dot pattern of bright and dark dots on a plane. When being transmitted through the spatial light modulator (SLM) 15, the signal light 12A is optically modulated so as to include a data signal component. The signal light 12A including the dot pattern signal component passes through a Fourier transform lens 16 arranged away from the spatial light modulator (SLM) 15 by a focal distance f of the lens 16, thereby the dot pattern signal component is transformed by Fourier transform. Thus, the signal light 12A is converged on a position in a recording medium 5. On the other hand, the recording reference light 12B obtained by splitting by the beam splitter 13 is directed to the inside of the recording medium 5 by a mirror 18 and a rotatable mirror 19, and intersects with the optical path of the signal light 12A in the recording medium 5 so as to form an optical interference pattern. The entire optical interference pattern is recorded in the recording medium 5 as a change of a refractive index.
In the above-mentioned manner, diffracted light from image data that has been illuminated with coherent collimated light is converged by the Fourier transform lens, thereby the image data is transformed into a distribution on a focal plane of the Fourier transform lens, i.e., a Fourier plane. The distribution obtained as a result of Fourier transform is made to interfere with a coherent reference light, so that interference fringes thus generated are recorded in the recording medium placed in the vicinity of the focal point of the Fourier transform lens. When recording of data of one page (hereinafter, simply referred to as a “page”) is finished, the rotatable mirror 19 is rotated by a predetermined amount and is translated by a predetermined distance, thereby changing an angle of incidence of the recording reference light 12B with respect to the recording medium 5. Then, data of the next page is recorded in a similar manner. By performing sequential recording in the above-described manner, angular multiplex recording is performed.
In reproduction, on the other hand, a dot pattern image is reproduced by performing inverse Fourier transform. In data reproduction, as shown in FIG. 1, the optical path of the signal light 12A is blocked by the spatial light modulator (SLM) 15, for example, thereby allowing only the reference light 12B to be incident on the recording medium 5. In reproduction, the position and angle of the rotatable mirror 19 are changed and controlled by a combination of rotation and translation of the rotatable mirror 19 so as to make the angle of incidence of the reference light the same as that of the recording reference light when a page to be reproduced was recorded. On the opposite side of the recording medium 5 on which the reference light 12B is incident, reproduction light that reproduces the optical interference pattern that has been recorded. The reproduction light is directed to an inverse Fourier transform lens 16A arranged away from the recording medium 5 by a focal distance f of the lens 16A, where the reproduction light is subjected to inverse Fourier transform. Thus, a dot pattern signal can be reconstructed. Moreover, the dot pattern signal is received by a photodetector 20 such as a charge-coupled device CCD, arranged at a position away from the lens 16A by the focal distance of the lens 16A, and is then converted into an electric digital data signal again. Then, the electric digital data signal is sent to a decoder 26, thereby original data is reproduced.
As described above, in order to record information in a certain volume in a recording medium with high density, the recording was conventionally performed for that volume of several cubic millimeters in a multiplexing manner using angular multiplexing or wavelength multiplexing. In such a recording or reproducing operation, signal light and/or reference light had to be fixed at a predetermined recording or reproducing position in the recording medium for a predetermined time period in accordance with the sensitivity of the recording medium and the photodetector. Thus, in recording of data, a position of interference between the signal light and the reference light was adjusted to the predetermined recording position in the recording medium and the recording of the data was then performed, while the recording medium was fixed. Subsequently, the position of the interference was moved and then next data was recorded. In reproduction, a position illuminated with the reference light was adjusted to the recording position at which the data was recorded and reproduction was then performed, while the recording medium was fixed. After the reproduction from that recording position was finished, the illuminated position was moved and then next data was reproduced.
Thus, the conventional technique had a problem that it was difficult to perform high-density recording and reproduction at a high speed. Moreover, there was another problem that in order to control a light beam in recording and reproduction, a high-precision paging control mechanism was required. This was disadvantageous to the size reduction of the system.
The present invention was made in view of the above, and the problems mentioned above are exemplary problems to be solved by the present invention. In other words, it is an object of the present invention to provide a recording and/or reproducing apparatus and a recording and/or reproducing method that can avoid a limitation on a recording or reproducing speed so as to enable high-speed and high-density recording and reproduction.
It is another object of the present invention to provide a recording medium that can avoid the aforementioned limitation on the recording or reproducing speed so as to enable high-speed and high-density holographic recording and reproduction.